1. Field of the Invention
The present invention relates to a high-pressure fuel pump for supplying a high-pressure fuel to an internal combustion engine.
2. Description of the Related Art
In a known high-pressure fuel pump, a tappet enclosed by a housing repeats vertical movements in alignment with a piston in accordance with the abutment or contact of the tappet with a cam while acting against the sliding resistance of an inner wall surface of the housing. The tappet does not basically rotate on its own axis, and hence the abutment of the tappet with the cam is always performed at the same locations or portions. As a result, wearing of the tappet progresses concentratedly only at those portions thereof at which the tappet is in line contact with the cam. In this case, there take place local wear of the tappet and resultant wear of the cam surface accompanying the tappet wear in accordance with a long period of use thereof, and if a prescribed amount of lift of the tappet provided by the cam profile is impaired, there might arise a situation where it becomes impossible for the piston to pressurize fuel to a desired pressure, thus leading to an inoperative condition of the high-pressure fuel pump.
In order to solve such an inconvenience, for instance, there has been known a method in which the outer peripheral surface of the tappet is formed into a special shape so that the sliding resistance between the tappet and the inner wall surface of the housing can be reduced, thereby facilitating accidental rotation of the tappet (a first patent document: Japanese patent application laid-open No. 2000-145572), or another method in which the cam profile is configured in such a manner that the abutment or contact position between the cam and the tappet is made apart from the axis of the tappet, thereby making it possible to obtain the self-rotating force of the tappet from a contact friction force generated during the rotation of the cam (a second patent document: Japanese patent application laid-open No. 2002-31017).
However, the high-pressure fuel pumps as constructed above have the following problems. That is, in the method described in the above-mentioned first patent document, it is difficult to form the outer peripheral surface of the tappet into a complicated configuration.
In addition, in the method described in the above-mentioned second patent document, a force acting on the piston in a diametral or radial direction thereof is generated due to eccentricity between the cam and the piston, thereby causing seizure of the piston.